1. Field of the Invention
The invention relates to the calibration of indicators which show the position of control rods in nuclear reactors. More particularly, it concerns a method of rod position indicator calibration which does not require shutting down of a reactor.
2. Description of the Prior Art
The core of a commercial pressurized nuclear reactor contains a fuel such as uranium 235. This core, which is within a pressure vessel, is cooled by the flow of a reactor coolant, such as water, which removes the heat generated by the nuclear reaction. The coolant flows into the pressure vessel, through the reactor core, out of the pressure vessel, through steam generators and then back into the pressure vessel. A secondary coolant medium, usually also water, is provided in heat transfer relationship within the steam generator with the reactor coolant. This secondary coolant is converted into steam in the steam generator and is thereafter used to produce electricity in a conventional steam turbine electrical generator system.
In such nuclear reactors, control rods are employed for controlling the power output and the power distribution within the core, as well as to shut down the nuclear reaction. Each control rod is actually comprised of a plurality of individual rods, a "rod cluster", which cluster is attached to a drive shaft for vertical movement with respect to the core. A drive mechanism moves the control rods into and out of the core.
The rate of heat generation in the reactor core is proportional to the rate of nuclear fission, which is determined by the neutron flux in the core. The control rods may consist of an alloy of silver-indium-cadmium that is a good absorber of neutrons.
Reactors of this type also employ a soluble absorber, which is incorporated in the coolant water. Boric acid is an effective soluble absorber of thermal neutrons. The boric acid can by itself, in sufficient concentration, keep the reactor shut down even if all of the control rods are fully withdrawn. The soluble boron controls all long term reactivity changes, whether caused by fuel depletion or other processes.
In a typical reactor, the control rods are arranged in several banks, that is, groups of rod clusters that ordinarily are moved simultaneously. Each control rod of each bank is intended to be lowered into the core to the same extent as all of the other rods in the same bank. When a rod is displaced from other rods of its bank in the vertical, axial, sense, the rod is said to be misaligned.
The control rods (i.e. the rod clusters) are mounted on drive shafts which have equally spaced notches or grooves for engagement by two axially spaced sets of latches. Thus the position of a rod can be determined by counting the number of grooves or steps by which a rod is lifted from its fully inserted position. The position of rods can also be checked by performing a flux mapping of the reactor core. When a rod appears to be misaligned, the operator must take action, either to correct the rod position, or to determine that the position is correct and then to calibrate the rod position indicator so that the rod position is shown accurately.
Besides the step counters to indicate the position of control rods, there is also a rod position indication (RPI) system for monitoring the position of each rod. The system includes a rod position detector for each rod, which produces an electrical voltage signal that is inversely proportional to the extent of insertion of the rod into the reactor core. Thus the voltage is zero when the rod is fully inserted, and has its maximum value when the rod is fully withdrawn. This signal is processed and transmitted to a control panel so that an operator can determine whether the rods are properly positioned.
After the rod position indication system has been initially calibrated, with the reactor in a shut down condition, the voltage signal is accurate to within .+-.5% of the full length of rod travel.
Each rod position detector is a variable transformer consisting of primary and secondary transformer coils "stacked" vertically around the rod drive shaft, which serves as a "core" of the transformer. With a controlled current signal applied to the primary transformer coil windings, the vertical position of the control rod drive shaft determines the extent of electromagnetic coupling with the secondary transformer coils, and thus produces a secondary voltage which is directly proportional to the extent to which the shaft is inserted through the coils, i.e. directly proportional to the extent to which the rod is withdrawn from the reactor core.
When it is indicated that a control rod position is further from the demand position where it is meant to be than allowed by the .+-.5% error band, it is possible that the rod is misaligned. Another possibility is that the RPI system is not properly calibrated. In the past, when a rod or rods were indicated to be outside the .+-.5% band, the procedure has been to fully insert all of the control rods, shutting down the nuclear reaction, and then lift the rods, step by step, calibrating the voltage shown by the rod position indicators against the number of steps counted.
Obviously shutting down a power generating reactor is an extreme measure; time is required to restart the reactor after shutdown, and the whole procedure is very costly. The present invention provides a method for calibration of the rod position indication system while the reactor continues to operate at a somewhat reduced level. At such a reduced power level, the generator still remains on the line, continuing to produce electricity, whereas when the reactor is shut down, the generator is off line, and restart is more complicated. The method still allows for the safeguard of resort to conventional shutting down of the nuclear reaction.
More specifically, and by way of example, in a conventional pressurized light water reactor of the type produced by Westinghouse Electric Corporation, there are 53 control rods, each comprising a cluster of 20 individual absorber rods. For simplicity, each cluster of 20 absorber rods, which move together, is referred to simply as a control rod. Some of the control rods are shutdown rods used primarily to shut down the reactor, as in the case of an accident. The remainder of the control rods are used for both shutdown and control. They are combined into four rod banks which operate sequentially, when one bank has been partially withdrawn, the next bank starts to withdraw, and so on until all the rod banks are withdrawn. The same overlapping sequence is followed during rod insertion, except during emergency shutdowns, when these rods fall rapidly into the reactor core, as do the shutdown rods. There is a rod bottom position detection system for each rod which signals that the rod is fully inserted.
Rod positions are detected by the transformer coils mounted around the rod drive shafts which produce voltage signals presented to the operator for visual monitoring. Thus rod position is shown as a voltage signal proportional to rod position, on a voltmeter. A reading of 3.45 volts corresponds to a rod withdrawn a full 230 steps (144 inches), each step corresponding to the uniform 5/8 inch spacing between the grooves or notches of the rod drive shafts, and a reading of 0.00 volts indicates a fully inserted rod, i.e a rod on the bottom. Rod position is also shown as counted in terms of steps. There is also a separate bottom indicator for each control rod, which shows that the rod is essentially at the bottom of its travel.
The actual indicated rod position versus the demand position for the rod is logged at least once per shift by the operator. If two rods in a given bank deviate from each other by a preset amount or if any rod deviates from its demanded position by a preset amount, corrective action is required. The overall accuracy of the rod position indication system is about .+-.5% of full rod travel, or .+-.7.2 inches or 12 steps. Thus a rod is considered to be misaligned if it is not within 12 steps of its demanded position. (When a rod is almost fully withdrawn, say more than 210 steps withdrawn, a deviation of 17 steps in the positive sense from demand position is considered tolerable.)
When a rod is indicated to be misaligned, though in fact it is not, the rod position indication (RPI) system requires calibration. In the past calibration has required the plant to be shut down.